The present invention relates to the general field of nozzles for low-pressure turbines of turbomachines. More particularly it relates to a top platform and to a bottom platform serving as supports for the fixed vanes of such nozzles.
In a turbomachine, the nozzle of the low-pressure turbine is disposed downstream from the high-pressure turbine in the flow direction of the gas passing through the turbomachine. FIG. 5 is a diagram showing the junction between the high- and low-pressure turbines in a conventional turbomachine. In this figure, the high-pressure turbine 100 has a row of moving blades 102 spaced apart circumferentially around a longitudinal axis 104 of the turbine. As represented by arrow 106, the flow of gas leaving the high-pressure turbine flows towards the nozzle 108 of the low-pressure turbine. The nozzle is constituted in particular by a plurality of fixed vanes 110 extending radially between a top annular platform 112 and a bottom annular platform 114 acting as supports. These fixed vanes 110, which are likewise spaced apart circumferentially around the longitudinal axis 104, serve to direct the flow of gas coming from the high-pressure turbine at an appropriate angle and at an appropriate speed. The inside surfaces 116, 118 of the top and bottom platforms 112, 114 for supporting the fixed vanes 110 define between them an aerodynamic channel 120 through which the gas flows.
In order to improve the efficiency of the low-pressure turbine, it is known to lengthen the aerodynamic channel for the flow of gas passing through the turbine by spacing the leading edges of the fixed vanes of the low-pressure turbine apart from the trailing edges of the moving blades of the high-pressure turbine. Nevertheless, that configuration requires the aerodynamic channel to be made continuous upstream from the platforms so as to limit head losses between the high-pressure turbine and the nozzle. A known solution to that problem consists in providing the platforms of the low-pressure turbine nozzle with ferrules extending upstream in line with the bottom and top platforms. These ferrules are generally fixed to a casing of the turbomachine via bolt fasteners. That solution nevertheless presents drawbacks associated in particular with the undesirable extra weight represented by such ferrules and their connections. In addition, in the absence of any specific cooling system, those ferrules tend to heat up quickly which degrades the mechanical behavior and the quality of continuity in the aerodynamic channel. In practice, the mechanical behavior of the ferrules is observed to deteriorate quickly and steps are seen to be formed in the inside surfaces thereof, which steps are particularly harmful in terms of the efficiency and the lifetime of the low-pressure turbine.